1. Field of the Invention
This invention relates to birefringent optical multiplexers having a flattened bandpass. Accordingly, it is a general object of this invention to provide new and improved multiplexers of such character.
2. General Background
Wavelength-Division Multiplexing (WDM) can be used to combine two or more optical wavelength carriers in a single optical path, in such a manner that they can be completely separated (de-multiplexed) subsequently with their respective data contents intact. Some WDM designs, including this instant invention, can be used for optical duplexing to provide two sets of signals for propagation in opposite directions through the same fiber.
An advantage of Wavelength-Division Multiplexing is that the total data capacity of a system can be multiplied by a factor of two or more beyond the limitations imposed by the speed of the electronics or the dispersion of an optical fiber, since each channel of a wavelength-division multiplexing system can be modulated at full capacity without affecting other channels in the same fiber.
Our parent application describes the relationship of our basic birefringent multiplexer system to previous designs. In brief, the known prior art optical multiplexer designs use either diffraction gratings or dichroic filters as the basic means of combining and/or separating various wavelength channels.
Our design as set forth in the parent application, in contrast, utilizes birefringent optical filtering in combination with a polarization insensitivity feature and the ability to tune the passband over a very wide range.
Disadvantageously, currently known available WDM components, utilizing dichroic filters or gratings, are severely limited by very wide channel spacing, much wider than the very small range over which fiber dispersion is insignificantly small. Thus, optimal use of the fiber's high bandwidth capability can only be achieved on one of the channels. Attempts to achieve closer spacings have resulted in higher temperature sensitivity, higher losses, difficult mechanical tolerances, and low yields for complex coating designs.